1. Field of the Invention
The present invention relates generally to methods and aqueous acid solutions for acidizing wells.
2. Description of the Prior Art
Subterranean oil containing formations penetrated by well bores are often treated with aqueous acids to stimulate the production of oil therefrom. One such treatment generally referred to as “acidizing” involves the introduction of an aqueous acid solution, usually an aqueous hydrochloric acid solution, into a subterranean formation under pressure so that the acid solution flows through the pore spaces of the formation. The acid reacts with the acid soluble materials contained in the formation thereby increasing the size of the pore spaces and increasing the permeability of the formation. Another production stimulation treatment known as “fracture-acidizing” involves the formation of one or more fractures in the formation and the introduction of an aqueous acid solution into the fractures to etch the fracture faces whereby channels are formed therein when the fractures close. The acid also enlarges the pore spaces in the fracture faces and in the formation.
Acidizing and fracture-acidizing solutions typically contain from about 15% to about 28% hydrochloric acid which causes corrosion of metal surfaces in pumps, tubular goods and equipment used to introduce the aqueous acid solutions into the subterranean formations to be treated. As a result, metal corrosion inhibiting additives have been developed and included in the aqueous acid solutions.
In wells that contain acid sensitive oils such as asphaltenic oils, sludges and water-in-oil emulsions are produced when the oils are contacted with aqueous acid solutions. To combat this problem, anti-sludging/de-emulsifying agents have heretofore been utilized in the aqueous hydrochloric acid solutions.
The most commonly used anti-sludging/de-emulsifying agents include the anionic surfactant dodecylbenzenesulfonic acid. However, the anionic nature of dodecylbenzenesulfonic acid brings about incompatibilities with the cationic hydrochloric acid corrosion inhibitors. The cationic hydrochloric acid corrosion inhibitors are generally included in aqueous acid solutions utilized in wells penetrating subterranean zones having high temperatures, e.g., 250° F. and higher. However, a problem that has been encountered with the use of the high temperature performing cationic corrosion inhibitors is that the quaternary ammonium chloride ions in the corrosion inhibitors react with the dodecylbenzenesulfonic acid in the anti-sludging/de-emulsifying agent resulting in precipitation and loss of acidizing performance. As a result, hydrochloric acid stimulation procedures in wells containing sludging and emulsifying oils is limited to temperatures in which non-cationic corrosion inhibitors can be used, i.e., to temperatures less than about 250° F. to thereby avoid precipitation, etc. Another disadvantage is that when hydrochloric acid cannot be used due to high temperatures and sludging and emulsifying oils, less effective organic acids must be employed.
Thus, there is a need for improved methods and aqueous acid solutions for acidizing subterranean zones containing sludging and emulsifying oils which do not include anionic anti-sludging/de-emulsifying agents that react with cationic corrosion inhibitors at high temperatures.